Caligula, formally Gaius Caesar Augustus Germanicus, became Roman emperor in 37 CE after Tiberius's death. His descent from Germanicus and Augustus helped secure popular enthusiasm, and the support of the praetorian prefect Macro eased his accession. He began by honouring dead relatives, recalling exiles and distributing money promised to the people and the guard. These gestures presented his rule as a welcome departure from Tiberius's final years.

Relations with the governing elite soon worsened. Caligula eliminated potential rivals, including Tiberius Gemellus, and asserted authority in ways that exposed the monarchy beneath the traditional language of the principate. His expenditure on spectacles and building projects, combined with demands for revenue, generated resentment. Ancient writers linked a serious illness to a sudden change in conduct, but their narratives do not justify a confident modern medical diagnosis.

In 39 he travelled to the northern frontier, where the discovery or allegation of conspiracy led to executions and the exile of his sisters. Accounts of the subsequent military activity and preparations concerning Britain are contradictory and often mocking. His dealings with client rulers also altered Rome's relationships in the eastern Mediterranean. A particularly dangerous dispute arose when he ordered an image of himself placed in the Jerusalem Temple. Resistance, intervention by Herod Agrippa and delaying action by the governor of Syria prevented the order from being carried through before his death.

In January 41 officers of the Praetorian Guard, including Cassius Chaerea, assassinated him in the palace complex. Claudius emerged as emperor. The surviving accounts are overwhelmingly hostile and contain stories shaped by conventions about tyrants. Those stories require caution, but the executions, court conflict and assassination reveal the instability of a system in which immense personal authority lacked an accepted peaceful means of removal.